1. Field of the Invention
The invention relates to a thermoplastic polymer blend comprising thermoplastic starch as well as a method for producing a thermoplastically deformable, biologically degradable polymer blend, that is shape-resistant in water, on the basis of native starch, synthetic polymers, for example, aliphatic polyesters and their copolymers, polyvinyl acetate (PVAc), polyvinyl alcohol (PVOH) and other, preferably biologically degradable synthetic polymers with addition of a hydrolysis component on the basis of PVAc as well as water or/and lower polyfunctional alcohols by reactive extrusion, preferably in double screw extruders. The reaction product can be processed as a function of the product composition by means of conventional processing machines for thermoplastics to injection molded, deep-drawn, and blow molded parts as well as foils with adjustable service value properties, for example, shape resistance in water and biological degradability. The polymer blend according to the invention is also used as a raw material for fibers as well as material for melt film coatings.
2. Description of the Related Art
In recent years, several methods for manufacturing and forming thermoplastic starch (TPS), alone or in the form of a polymer mixture or polymer melt or polymer blend, have become known. This research has been carried out with the goal of making available new or broader fields of application for renewable raw materials.
It is known to hydrolyze the grainy structure of native starch first with defined proportions of water or/and lower polyfunctional alcohols, such as ethylene glycol, propylene glycol, glycerol, 1,3-butanediol, diglyceride, corresponding ethers, but also components such as dimethylsulfoxide, dimethylformamide, dimethylurea, dimethyl acetamide and/or other additives to a thermoplastic material by thermo-mechanical hydrolysis in conventional double screw extruders.
The service value properties of the extruded starch material and of the products produced therefrom is minimal. In particular, the material is very hydrophilic, ages during storage, and exhibits processing problems. For this reason, synthetically obtained water-resistant polymers, such as, for example, polyethylene, polypropylene, or aliphatic co-polyesters, aliphatic-aromatic co-polyesters, polyester amides, polyester urethanes, water-resistant starch/cellulose derivatives and/or other mixtures are added to the starch. However, this causes the problem that the compatibility among the polymer components is unsatisfactory. Moreover, the biological degradability and also the cost structure become less favorable.
The current state of the art is documented extensively in the form of printed materials. Reference is being had in this connection to the publication by R. F. T Stepto et al. “Injection Molding of Natural Hydrophilic Polymers in the Presence of Water” Chimia 41 (1987) No. 3, pp. 76-81, and the literature cited therein as well as, in an exemplary fashion, to the patents DE 4116404, EP 0327505, DE 4038732, U.S. Pat. No. 5,106,890, U.S. Pat. No. 5,439,953, DE 4117628, WO 94/04600, DE 4209095, DE 4122212, EP 0404723, or EP 407350.
In DE 40 32 732 the starch, plasticized with water and glycerol, is processed primarily with polyvinyl acetate to a polymer mixture in an extruder. The extruded material has an improved water resistance in comparison to TPS. With a higher starch proportion the extruded material and the bottles produced therefrom turn yellow to brownish. The amount of starch is thus limited to less than 50%.
A neutral to slightly acidic component of polyvinyl acetate and water glass has also been suggested already (DE 195 33 800) with which a polymer mixture of starch and a hydrophobic polymer, for example, polyvinyl acetate, can be extruded. The component is produced of water glass and polyvinyl acetate during the extrusion process with intensive mixing action. Optionally, acetic acid is added in order to neutralize not yet reacted water glass after saponification of PVAc with water glass. It was found that already minimal additions of this component results in a significant quality improvement of the extruded material and of the products produced thereof. Significantly more native starch can be used in comparison to the prior art without discoloration or with only minimal discoloration while maintaining or improving the shape resistance in water. Apparently, the component contributes to making the two phases, i.e., the hydrophilic thermoplastic starch and the hydrophobic polymer which, in fact, are immiscible, miscible to a certain degree.
Further experiments have shown that the quality of the end product with respect to shape stability in water and strength must be improved even further. In particular, it was impossible to produce thin foils under approximately 300 μm thickness.
It has already been suggested (DE 197 50 846) to produce a component of polyvinyl acetate and alkali water glass and to extrude this component together with starch and a hydrophobic polymer, for example, polyvinyl acetate, to a high-quality polymer mixture. For producing the component, polyvinyl acetate was saponified to a hydrolyzation degree of up to 90% in the presence of catalytic amounts of low-molecular organic mono-, di- and trihydroxy components (for example, methanol, ethanol, ethylene glycol, glycerin) and with continuous addition of alkaline-reacting components and of the alkali silicate in a batch process.
It was found that with this component, which obviously acts as a compatibility agent, polymer mixtures with high starch proportions can be extruded in analogy to DE 195 33 800. The products produced of this polymer mixtures have a significantly greater quality in regard to several parameters. In particular, foils of a thickness of less than 100 μm can be produced.